Bending spring element comprising a fibre reinforced material

Abstract

The invention relates to a flexible spring element (1) which is made of a fibre-plastic composite material (4, 22) and has a cover layer (2, 3) made of a first fibre-plastic composite material (4) on each of two outer sides (9, 10) lying opposite one another, fibres (5) being aligned parallel relative to one another at least in bundles within the cover layers (2, 3) and running parallel to the outer side (9, 10) associated with the respective cover layer (2, 3). The flexible spring element (1) has at least one curved portion (13, 14), in which a centre plane (8) of the unloaded flexible spring element (1) running with equal spacing between the two cover layers (2, 3) runs in a curved manner in a longitudinal direction (7) of the flexible spring element (1). The flexible spring element (1) also has at least one longitudinal portion (15, 16, 17) in which the centre plane (8) of the unloaded flexible spring element (1) either has no significant curvature or has a reversal of curvature. In the at least one curved portion (13, 14) the flexible spring element (1) has a spacing extending element (20, 21) which is arranged between the two cover layers (2, 3) and is made of a different material than the two cover layers (2, 3). The material of the spacing extending element (20, 21) may be a second fibre-plastic composite material (22) having short and undirected fibres (23).

Description

[0001] The invention relates to a flexural spring element made of a fiber-plastic composite material, wherein the flexural spring element has a cover layer made of a first fiber-plastic composite material on each of two opposite outer sides, wherein fibers within the cover layers are aligned parallel to one another at least in bundles and run parallel to the outer side assigned to the respective cover layer, wherein the flexural spring element has at least one curved section in which a center plane of the unloaded flexural spring element, running at an equal distance between the two cover layers, is curved in a longitudinal direction of the flexural spring element, wherein the flexural spring element has at least one longitudinal section in which the center plane of the unloaded flexural spring element either has no significant curvature or a reversal of the curvature,wherein, during a proper deflection of the flexural spring element, a cover layer section arranged outwardly in the curved section is subjected to tensile stress and a cover layer section opposite and inwardly directed in the curved section is subjected to compressive stress, and wherein the flexural spring element has exclusively the two cover layers in the at least one longitudinal section, and that the flexural spring element has, in the at least one curved section, a spacing extension element arranged between the two cover layers and made of a different material than the two cover layers, wherein the flexural spring element has at least two curved sections separated from one another by a longitudinal section and curved in different directions, so that the center plane has an S-shaped profile across these two curved sections.

[0002] A spiral spring element can be designed as a single piece and form a spiral spring or a spiral spring device. It is also possible for multiple spiral spring elements to be combined and operatively connected to one another to form a spiral spring device. A single spiral spring element can, for example, be beam-shaped and have a straight or slightly curved profile. Spiral spring elements are also known that have a complex curved profile and, for example, a C-shaped or S-shaped configuration, or several alternating and meandering curves.

[0003] Spiral spring elements can be made from various materials. Conventional spiral spring elements, for example, are often made from a suitable spring steel. Such spiral spring elements can be manufactured cost-effectively, and the shape of the spiral spring elements can be adapted to the specific task. Spiral spring elements made from spring steel or another suitable metal are robust and exhibit advantageous spring properties. Highly robust spiral spring elements made from metal are shown and described, for example, in US 2018 / 0216678 A1.

[0004] It is also known and has already been tested for various applications to manufacture spiral spring elements from a suitable fiber-reinforced plastic composite material. Typically, a fiber-reinforced plastic composite material is used, in which quasi-continuous fibers are embedded in a suitable plastic matrix material. The arrangement and alignment of the individual fibers, which are primarily responsible for transmitting tension and pressure along the spiral spring element, can promote advantageous spring properties. Compared to metal spiral spring elements, spiral spring elements made from a fiber-reinforced plastic composite material can be lighter and offer better resistance to environmental conditions, particularly to moisture. A spiral spring device composed of several spiral spring elements is described in DE 10 2015 012 334 B3.Bending spring elements made of a suitable fiber-reinforced plastic composite material can be used advantageously in motor vehicles.

[0005] However, the production of spiral spring elements from a fiber-reinforced plastic composite material is often associated with high manufacturing effort and, consequently, high production costs. Particularly for spiral spring elements with a varying thickness, the effort required to prepare a large number of prefabricated strips of fiber-reinforced plastic composite material, or prepregs, of different lengths, their arrangement in a mold, and their subsequent pressing is quite considerable and often decisive in terms of production costs. Such a spiral spring device is described, for example, in DE 10 2008 006 411 A1. Other means for influencing the spring properties of spiral spring elements made from a fiber-reinforced plastic composite material are also known, for example, from WO 1985 / 000207 A1.

[0006] A bending spring element designed as a substantially flat leaf spring is described, for example, in US Pat. No. 3,968,958. The assembly of the pre-assembled prepregs of different lengths, and in particular their arrangement in a mold, as well as their fixation during the pressure-curing process of the prepregs in the mold, involves considerable effort and can often only be performed manually. The shorter, additional middle layers arranged between the outer cover layers further reinforce a central section of the leaf spring.

[0007] In a torsion spring element with a curved section where the course of the torsion spring element changes by at least more than 90°, but in many practical applications by approximately 180°, the curved section is often thicker than an adjacent longitudinal section in which the course of the torsion spring element does not change, or at least not significantly. The torsion spring element is subjected to greater stress in the curved section when a force is applied transversely to the course in the longitudinal direction.In the case of a proper deflection of the bending spring element by the acting force, a cover layer section arranged outwards in the curved section is subjected to tensile stress and a cover layer section opposite and inwards in the curved section is subjected to compressive stress, while the cover layer sections in an adjacent longitudinal section of the bending spring element are subjected to less tensile or compressive stress.

[0008] If the torsion spring element is designed with a uniform thickness or with a uniform spacing between the two cover layers, the thickness of the torsion spring element must be adapted to the tensile and compressive loads that can occur at the maximum intended force acting on the curved section of the torsion spring element. In an adjacent longitudinal section, a significantly lower load occurs, so that the torsion spring element is oversized with a uniform thickness in this longitudinal section. The weight of the torsion spring element and the material requirements would then be excessive, which is considered disadvantageous, particularly with regard to the typically desired lightweight construction using fiber-reinforced plastic composite materials.

[0009] EP 0 005 916 discloses a flexural spring element designed as a leaf spring, in which an insert made of another plastic composite material with short, non-directional fiber pieces is arranged between two outer cover layers made of a plastic composite material with longitudinally directed fibers. The insert extends almost the entire length of the flexural spring element and is thicker in a central region than toward the respective ends. EP 0 005 916 discloses a flexural spring element with the features of the preamble to claim 1.

[0010] It is therefore considered an object of the present invention to design a bending spring element made of a plastic composite material in such a way that the bending spring element can be produced cost-effectively, can be easily adapted in different sections to the stresses that usually occur and can have the most advantageous spring properties possible.

[0011] This object is achieved according to the invention by a bending spring element having the features of claim 1.

[0012] It has been shown that particularly advantageous spring properties can be achieved with minimal material and manufacturing effort by simply arranging spacing extension elements in at least one curved section, and preferably in several curved sections, of the flexural spring element. These elements can be used to increase the thickness of the flexural spring element measured transversely to the longitudinal direction of the flexural spring element. The deformation forces resulting from the bending stress on the flexural spring element under normal loading can be particularly advantageously absorbed by the spacing extension element in a thickened curved section and converted into spring energy.By increasing a distance of the cover layers measured transversely to the longitudinal direction and thus a distance of the cover layers to the center plane and the neutral fiber running therein, the bending spring element has a greater resistance to deformation in the curved section than in an adjacent longitudinal section, without having to arrange additional center layers between the cover layers for this purpose, which extend only over the curved section.

[0013] According to the invention, the spacing extension element arranged in the curved section between the two cover layers has a continuously varying thickness in the longitudinal direction, so that the spacing extension element becomes continuously thicker starting from a tapered first end and has a maximum thickness in a central region, then increasingly tapers towards the opposite second end and also tapers again. In this way, abrupt changes in thickness in the flexural spring element can be avoided, which experience has shown can lead to load peaks and often very high and possibly excessive stress during intended use of the flexural spring element.

[0014] It is advantageous if the spacing extension element is made of a material that is as shear-resistant and stiff as possible. It is generally conceivable for the spacing extension element to be made of wood or a suitable plastic material, for example. The spacing extension element can be prefabricated or manufactured in advance, with the spacing extension element expediently already having a shape that is adapted to the shape or profile of the unloaded flexural spring element within the curved section.

[0015] Spiral spring elements with at least one S-shaped spring section, and in particular spiral spring elements with two or more adjacent S-shaped spring sections, enable a particularly advantageous combination of high spring force with the smallest possible space requirement for many applications. Several similar or identical spiral spring elements can be combined to form a spiral spring device that combines advantageous spring properties with high resistance to environmental influences and a low weight. A spiral spring element according to the invention or a spiral spring device combined from several spiral spring elements is therefore also particularly suitable for use as a spring element in motor vehicles.

[0016] According to an advantageous embodiment of the inventive concept, the material of the spacing extension element is a second fiber-reinforced plastic composite material with fibers whose length is less than 30 mm, preferably less than 10 mm, and particularly preferably less than 1 mm. The second fiber-reinforced plastic composite material with such short fibers can be processed particularly advantageously and cost-effectively and shaped into the desired shape for the spacing extension element. The individual fibers cannot transmit tensile or compressive forces over large areas, although this has proven unnecessary.By arranging a spacing extension element between two outer cover layers, a thickness of the flexural spring element, measured transversely to a longitudinal direction of the flexural spring element, can be adapted to the intended bending stress of the flexural spring element in the curved section and can be designed to be significantly thicker than the total thickness of the two cover layers. The spacing extension element, made from a second fiber-reinforced plastic composite material, can have a low dead weight. The additional second fiber-reinforced plastic composite material of the spacing extension element allows the two significantly more cost-intensive cover layers of the flexural spring element to be comparatively thin in the curved section and to be adapted to the intended or maximum expected tensile and compressive stress of the cover layers of the flexural spring element within the curved section.

[0017] It has been found that the flexural spring element has particularly advantageous spring properties when the fibers in the second fiber-reinforced plastic composite material are arranged randomly. Such a random arrangement of the fibers embedded in the matrix material of the second fiber-reinforced plastic composite material is also referred to as random fibers. In many applications, the more homogeneously the fibers are distributed in the second fiber-reinforced plastic composite material, the more advantageous the spring properties can be. The shorter the length of the individual fibers in the second fiber-reinforced plastic composite material, the easier it is to achieve a homogeneous distribution of the fibers in the spacing extension element according to the invention, as well as a homogeneously distributed alignment of the individual fibers relative to one another.

[0018] According to a particularly advantageous embodiment of the inventive concept, the second fiber-reinforced plastic composite material comprises a plastic matrix material that matches the two cover layers. By using the same plastic matrix material for the cover layers and for the spacer element arranged between the cover layers, a material-to-material and seamless connection of the spacer element to the adjacent cover layers on both sides can be achieved. This prevents, and at least impedes, unintentional detachment of the cover layers from the spacer element, even under high loads on the flexural spring element.

[0019] A spacer element can be manufactured by inserting a pasty starting material between prefabricated and, if necessary, preformed cover layers. Depending on the material used, the spacer element can also be manufactured using conventional injection molding processes. The cover layers can then be bonded together with the spacer element arranged between them, thus producing the desired bending spring element. The spacer element can also be inserted into the curved section between the cover layers already arranged in a mold using conventional injection molding processes. It is also conceivable for the spacer element to be manufactured in a separate production step, separate from the cover layers.The cover layers can then be connected together with the spacing extension element arranged between them and formed and solidified into the desired bending spring element.

[0020] Optionally, it is provided that the center plane in the at least one curved section of the spiral spring element has a change in direction of more than 90°, preferably more than 150°, and particularly preferably approximately 180°. In the case of a curved section whose course changes in the longitudinal direction by more than 150° and preferably by approximately 180°, a force applied transversely to the two end regions of the curved section can be particularly effectively absorbed by a proper deformation of the spiral spring element within the curved section, and a high spring restoring force and thus a strong spring effect can be generated with a small space requirement.

[0021] The cover layers exhibit particularly advantageous properties when the first fiber-reinforced plastic composite material of the two cover layers comprises fibers unidirectionally aligned in the longitudinal direction of the bending spring element. Due to the unidirectional and longitudinally aligned fibers, the cover layers can absorb particularly high tensile and compressive forces. The cover layers can comprise fibers or fiber bundles that run either parallel to the longitudinal direction or, preferably, at an acute angle to the longitudinal direction. Suitable fibers can be, for example, glass fibers, carbon fibers, ceramic fibers, basalt fibers, metal fibers, or even natural fibers. A suitable matrix material can be a plastic material adapted to the respective fibers, such as a suitable duromer, elastomer, or thermoplastic.The spacing extension element can have fibers made of the same material or of a different material as the cover layers.

[0022] Optionally, the first fiber-reinforced plastic composite material comprises fibers whose length extends longitudinally across the entire flexural spring element. Tensile or compressive loads acting on the cover layers are thus distributed across the entire flexural spring element by the virtually continuous fibers, reducing the risk of breakage in the event of excessive loading.

[0023] An exemplary embodiment is explained in more detail below, which is shown schematically in the drawing. It shows: Figure 1 a schematic sectional view of a bending spring element with an S-shaped course, Figure 2 a schematic sectional view of the Figure 1 shown bending spring element along a line II-II in Figure 1 , Figure 3a schematic sectional view of the Figure 1 shown bending spring element along a line III-III in Figure 1 , Figure 4 a schematic sectional view of a bending spring element according to the invention with several curved sections, which is intended and suitable for use in a motor vehicle, and Figure 5 a schematic sectional view of a differently designed bending spring element, which is also intended for use in a motor vehicle.

[0024] One in the Figures 1 to 3The bending spring element 1, shown in various views, has two cover layers 2, 3, each made of a first fiber-plastic composite material 4. In the two cover layers 2, 3, endless fibers 5 are arranged in a matrix plastic material 6 such that the fibers 5 extend in a longitudinal direction 7 across the entire bending spring element 1. The longitudinal direction 7 corresponds to the course of a center plane 8, which runs between two outward-facing outer sides 9, 10 of the two cover layers 2, 3, each at an equal distance from the two outer sides 9, 10. In the exemplary embodiment shown, the course of the center plane 8 also corresponds to the course of a neutral fiber, which is not loaded when a force F is applied as intended, transversely to the longitudinal direction 7, to both end regions 11, 12 of the bending spring element 1.However, it is also conceivable that, for example, the two cover layers 2, 3 do not have the same thickness, so that the center plane 8 does not necessarily have to correspond to the course of the neutral fiber.

[0025] The central plane 8 has an S-shaped profile. The spiral spring element 1 has two curved sections 13, 14 with a curved profile changing by approximately 180°, each arranged between two longitudinal sections 15, 16, 17. The two end regions 11, 12 are each formed by a longitudinal section 15, 17, in which the spiral spring element 1 has an approximately straight profile of the central plane 8. Between the two curved sections 13, 14, a longitudinal section 16 is also formed, in which the central plane 8 runs approximately straight and has a reversal of curvature from the first curved section 13 to the second curved section 14.

[0026] In the longitudinal sections 15, 16 and 17, the two cover layers 2, 3 are connected to each other in direct contact, as shown schematically in Figure 3 The fibers 5 embedded in the matrix plastic material 6 of the cover layers 2, 3 run essentially parallel to the respective outer sides 9, 10 of the respective cover layer 2, 3 and extend in the longitudinal direction 7 over the entire bending spring element 1. The orientation of the individual fibers 5 is accordingly Figure 3 shown sectional view is perpendicular to the plane of the image and runs in the Figure 1shown sectional view within the plane of the image. When a force F acts on the bending spring element as intended, the fibers 5 are subjected to tensile stress essentially along the outer side sections 18 directed outward in the curved section 13, 14 and to compressive stress along the outer side sections 19 directed inward in the curved section 13, 14.

[0027] A spacing extension element 20, 21 is formed in each of the two curved sections 13, 14. The spacing extension element 20, 21 creates a larger spacing between the outwardly directed outer side sections 18 and the inwardly directed outer side sections 19 of the two cover layers 2, 3 within the curved sections 13, 14, thereby providing advantageous spring properties of the flexural spring element 1 in the region of the respective curved section 13, 14.

[0028] The two spacing extension elements 20, 21 have an approximately crescent-shaped configuration. According to the invention, the spacing extension element 20, 21 arranged in the respective curved section 13, 14 between the two cover layers 2, 3 has a continuously varying thickness in the longitudinal direction, so that the spacing extension element 20, 21 continuously increases in thickness starting from a tapered first end and has a maximum thickness in a central region, then increasingly tapers towards the opposite second end and also tapers again. In this way, abrupt changes in thickness in the flexural spring element 1 can be avoided, which experience has shown can lead to load peaks and often very high and possibly excessive stress during intended use of the flexural spring element 1.The two distance extension elements 20, 21 do not necessarily have to be designed symmetrically to the center plane 8.

[0029] Each of the two spacing extension elements 20, 21 is made of a second fiber-reinforced plastic composite material 22. The second fiber-reinforced plastic composite material 22 has the same matrix plastic material 6 as the first fiber-reinforced plastic composite material 4, so that the spacing extension elements 20, 21 bond firmly and homogeneously to the two adjacent cover layers 2, 3, without the formation of interfaces between the spacing extension elements 20, 21 and the adjacent cover layers 2, 3 that could potentially impair the mechanical strength of the flexural spring element 1.

[0030] Short fibers 23, preferably with a uniform length between 1 mm and 5 mm, are arranged in the matrix plastic material 6 of the second fiber-reinforced plastic composite material 22. The short fibers 23 are oriented randomly in the spacing extension elements 20, 21, so that a substantially homogeneous distribution of the short fibers 23 is present in the matrix plastic material 6 of the second fiber-reinforced plastic composite material 22, aligned in all directions.

[0031] The dimensions of the spacing extension elements 20, 21, particularly with regard to their respective thickness transverse to the course of the center plane 8, are dimensioned such that the flexural spring element 1 exhibits advantageous spring properties within the intended range of the typically occurring force application, and damage to the flexural spring element 1 is largely excluded. At the same time, the cover layers 2, 3 are also dimensioned such that the intended use of the flexural spring element 1 is possible over the intended period of use, while still requiring as little material as possible for the cover layers 2, 3 and the spacing extension elements 20, 21, so that the flexural spring element 1 according to the invention exhibits advantageous spring properties with a particularly low dead weight.

[0032] In the Figures 4 and 5In each case, a bending spring element 1 designed according to the invention is shown with more than two, or with a total of four curved sections 24. The individual curved sections 24 are arranged between longitudinal sections 15, 16, 17 adjacent to both sides. Each of the Figures 4 and 5 The exemplary bending spring element 1 has a plurality of S-shaped and merging bending spring sections. Arranged in each of the four curved sections 24 is a spacing extension element 25, which consists of the second plastic material 22 with the short and undirected fibers 23 embedded in the matrix plastic material 6.

[0033] The individual spacing extension elements 25 do not have to have a matching shape. Rather, the individual spacing extension elements 25 are adapted to the profile 7 of the respective bending spring element 1 or to the profile 7 of the respective curved section 24.

[0034] In the Figure 4 In the embodiment shown, the two end regions 11, 12 are aligned approximately parallel to each other and extend in opposite directions, whereby a force predetermined and intended by the fixing of the bending spring element 1 acts perpendicularly on the two end regions 11, 12. In the embodiment shown in Figure 5In the embodiment shown, the two end regions 11, 12 are also aligned parallel to one another and also run in opposite directions, but a intended force acts approximately in the direction predetermined by the alignment of the end regions 11, 12 or in a parallel alignment on the two end regions 11, 12.

Claims

1. Flexible spring element (1) made of a fibre-plastic composite material (4, 22), wherein the flexible spring element (1) has a cover layer (2, 3) made of a first fibre-plastic composite material (4) on each of two mutually opposed outer sides (9, 10), wherein fibres (5) within the cover layers (2, 3) are aligned parallel relative to one another at least in bundles and run parallel to the outer side (9, 10) associated with the particular cover layer (2, 3), wherein the flexible spring element (1) has at least one curved portion (13, 14, 25), in which a centre plane (8) of the unloaded flexible spring element (1) running with equal spacing between the two cover layers (2, 3) runs in a curved manner in a longitudinal direction (7) of the flexible spring element (1), wherein the flexible spring element (1) has at least one longitudinal portion (15, 16, 17), in which the centre plane (8) of the unloaded flexible spring element (1) either has no significant curvature or has a reversal of curvature, wherein in the case of an intended deflection of the flexible spring element (1) a cover layer portion (18) arranged directed outwardly in the curved portion (13, 14, 25) is subjected to tensile loading and an inwardly directed opposing cover layer portion (19) in the curved portion (13, 14, 25) is subjected to compressive loading, and wherein the flexible spring element (1) comprises exclusively the two cover layers (2, 3) in the at least one longitudinal portion (15, 16, 17), and wherein in the at least one curved portion (13, 14, 25) the flexible spring element (1) has a spacing extending element (20, 21) made of a different material than the two cover layers (2, 3) arranged between the two cover layers (2, 3), characterised in that the flexible spring element (1) has at least two curved portions (13, 14, 25) separated from one another by a longitudinal portion (15, 16, 17) and curved in different directions, so that the centre plane (8) has an S-shaped course over these two curved portions (13, 14, 25).

2. Flexible spring element (1) according to claim 1, characterised in that the material of the spacing extending element (20, 21) is a second fibre-plastic composite material (22) comprising fibres (23) of which the length is in each case less than 30 mm, preferably less than 10 mm and particularly preferably less than 1 mm.

3. Flexible spring element (1) according to claim 2, characterised in that the fibres (23) are arranged undirected in the second fibre-plastic composite material (22).

4. Flexible spring element (1) according to claim 2 or claim 3, characterised in that the second fibre-plastic composite material (22) has a matrix plastic material (6) matching the two cover layers (2, 3).

5. Flexible spring element (1) according to any one of the preceding claims, characterised in that the centre plane (8) in the at least one curved portion (13, 14, 25) of the flexible spring element (1) has a direction change of more than 90°, preferably of more than 150° and particularly preferably of approximately 180°.

6. Flexible spring element (1) according to any one of the preceding claims, characterised in that the first fibre-plastic composite material (4) of the two cover layers (2, 3) comprises fibres (5) aligned unidirectionally in the longitudinal direction (7) of the flexible spring element (1).

7. Flexible spring element (1) according to claim 6, characterised in that the first fibre-plastic composite material (4) comprises fibres (5) of which the length in the longitudinal direction (7) extends over the entire flexible spring element (1).

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